The present invention relates generally to method and apparatus for inspecting bottles and, more particularly, to a method and apparatus for inspecting bottles which employs sound waves which rupture flawed bottles but which leave unflawed bottles undamaged.
FIG. 1 shows a conventional "convenience" bottle having a side wall 24. The side wall 24, as illustrated in FIGS. 2 and 3, has an exterior surface 42 and an interior surface 44. The exterior surface 42 is typically somewhat rougher than the interior surface 44. The interior surface may be made smoother by a bottle-forming technique known as "firepolishing" which increases the strength of the bottle. FIG. 3 shows a fracture 46 in the exterior surface 42 of the bottle. A fracture 46 acts as a stress concentrater which significantly reduces the rupture strength of the bottle. The degree to which such a fracture 46 reduces the burst strength of the bottle varies with the depth of the fracture and the type of fracture. Table I illustrates data compiled by American Glass research showing the effect of different types of fractures on the breaking strength of soda lime glass bottles, the breaking load being indicated in pounds per square inch.
Long ASTM Bottle Surface Time 1- 1- Impact Condition of Bottle Load 20-min minute second &lt;1 ms Pristine-Inside of 45000 63750 75000 100500 150000 Bottle Fire Polished Pristine Molded 12000 17000 20000 26800 40000 Mild Abrasions 6000 8500 10000 13500 20000 Moderate Abrasion 2500 3400 4000 5400 8000 (produced by 320 Grit sand paper) Moderately Severe 2250 2850 3350 4500 5700 Abrasion (produced by 150 Grit sandpaper) Severe Abrasion 1700 2125 2500 3350 5000 (produced by Diamond Scratch) Deep Bruises in Glass 650 1275 1500 2000 3000 Cracks in Glass 470 640 750 1000 1500
From the above table it may be seen that even very mild abrasions reduce the breaking strength of pristine-molded glass bottles (without inside fire polish) by 50% and that abrasions as small as 0.0005 inches in depth (150 grif sand paper) reduce the strength of pristine-molded glass bottles by 86%. It will also be appreciated that such small surface abrasions may be very difficult to detect by visual inspection. In addition to surface abrasions, there are a number of other types of flaws which reduce the breaking strength of a glass bottle including: score fractures, impact fractures, glass impurity fractures, stress concentrators due to improper melt temperature or improper cooling and stress concentrators caused by non-homogeneous compositions of glass and due to non-uniform glass distribution in the mold.
It is desirable for quality control purposes to test glass bottles after manufacture to determine whether flawed bottles are being created in the manufacturing process. The method for testing bottles most widely used in the industry today is known as a "squeeze tester". U.S. Pat. No. 5,351,552 of Giometti, which is hereby incorporated by reference, discloses such a squeeze tester. Bottles to be tested are moved along a conveyor belt which supports each bottle at its base. The bottles move along a path extending between a static wall on one side and the wall of a large rotating wheel on the other side. A bottle moving along the path is squeezed between the static wall and the moving wall provided by the large wheel, rotating as it moves along this portion of the path. The large wheel is biased towards the static wall and applies a predetermined pressure to the side wall of a bottle as it rotates through this portion of the bottle path. The bottle squeezer thus applies a selected amount of pressure in a direction perpendicular to the side wall of the bottle. The amount of pressure applied is selected to be less than that required to break an unflawed bottle, but more than that required to break a flawed bottle. ("Flawed bottle" as used herein is a relative term, the severity of bottle fracture to be detected being decided by quality control personnel who set the loading of the bottle tester to a value slightly higher than the strength of a bottle having such a fracture.) A problem with bottle squeezers has been that shattering glass from a flawed bottle may become imbedded in the side wall of the rotating wheel of the squeezer. This embedded glass or "stone" may cause scoring of bottles passing through the squeezer resulting in the flawing of bottles which were originally undamaged. If the flaw is generated towards the end of the rotation of the bottle through the squeezer, it may not be exposed to the maximum pressure of the squeezer and thus may pass through the squeezer unruptured, even though it is now flawed and has reduced rupture strength. Also, due to the fact that the wheel of the squeezer has a very large circumference compared to the circumference of a bottle, many bottles may pass through the squeezer which do not come into contact with the glass fragment imbedded in the squeezer wheel. Thus, it may be difficult to detect whether an increase in the number of flawed bottles detected by the squeezer has been caused by glass embedded in the squeezer wheel or other outside causes such as defects in the mold, etc. Another problem with bottle squeezers is that they cannot be operated at more than about 300 bottles per minute.
The following patents also relate to bottle squeezers and are hereby specifically incorporated by reference for all that is disclosed therein: U.S. Pat. No. 3,702,563 issued Nov. 14, 1972 of Brady et al; U.S. Pat. No. 3,729,082 issued Apr. 24, 1973 of Federko; U.S. Pat. No. 3,765,231 issued Oct. 16, 1973 of Erb et al; U.S. Pat. No. 3,777,556 issued Dec. 11, 1973 of Zappia; U.S. Pat. No. 4,021,122 issued May 3, 1977; U.S. Pat. No. 4,077,254 issued Oct. 4, 1994 of Mercer, Jr. et al; U.S. Pat. No. 4,096,939 issued Jun. 27, 1978 of Riggs et al; and U.S. Pat. No. 4,479,582 issued Oct. 30, 1984 of Ducloux.